EP4548469A1

EP4548469A1 - A power converter system with a submodule including a hydrogen electrolyser unit or a fuel cell

Info

Publication number: EP4548469A1
Application number: EP23735675.3A
Authority: EP
Inventors: Nicklas Johansson; Weichi ZHANG; Jan Svensson; Chunming YUAN; Jiuping Pan; Vijesh JAYAN; Shih-Feng Chou
Original assignee: Hitachi Energy Ltd
Current assignee: Hitachi Energy Ltd
Priority date: 2022-07-01
Filing date: 2023-06-26
Publication date: 2025-05-07

Abstract

The present disclosure relates to a system (100) and a method for use with an electrolyzer unit or a fuel cell are disclosed. The system comprises a converter unit (120) comprising a plurality of serially-connected converter cell units (140). At least one of the plurality of serially-connected converter cell its comprises a first terminal (147) and a second terminal (149). The at least one of the plurality of serially-connected converter cell units is dedicated to individually provide a direct current to one electrolyzer unit (160a) or to receive power from at least one fuel cell (160b), connected between the first terminal and the second terminal. The system further comprises a control unit (170) configured to control the direct current output from the at least one of the plurality of serially-connected converter cell units based on a reference value for driving the electrolyzer unit or to receive power from the at least one fuel cell. The present disclosure further relates to a method for controlling such a system.

Description

A POWER CONVERTER SYSTEM WITH A SUBMODULE

INCLUDING A HYDROGEN ELECTROLYSER UNIT OR A FUEL CELL

Technical field

The present disclosure generally relates to the field of electrical power conversion and, in particular, to a method and a system for driving hydrogen electrolysis in electrolyzer stations and/or for receiving power through fuel cells. More specifically, the present disclosure relates to a system including a converter arrangement for converting an AC current into a DC current for use with at least one electrolyzer unit or at least one fuel cell and to a method for controlling such a system.

Background

In order to limit the impact and the negative effects of climate change, a reduction of emission of CO2 and other gases which contribute to global warming is needed. One way to de-carbonize sectors which are hard to reach with other methods is by using hydrogen, which is an energy carrier. One way of producing hydrogen is by electrolysis, which splits water into oxygen and hydrogen using electricity. By using electricity from renewable sources, so called “green hydrogen” can be produced.

Hydrogen may for example be produced to a large extent when there is an excess of renewable power, such as wind or solar, and the electricity prices are low. There are several electrolyzer technologies that can be used in order to produce hydrogen. Regardless of the chemical reaction used in the process, the systems normally include at least one power electronic converter transforming an AC current from the electrical power grid to a DC current.

One example used today for conversion of the AC current is a 12-pulse thyristor rectifier. This solution is however problematic and brings challenges, for example since harmonics on the AC side will vary with varying electrolyzer load. There is therefore a need for harmonic filters to be used together with the thyristor rectifiers. A further issue with the use of thyristor rectifiers is that the reactive power varies with the produced hydrogen, which needs to be solved by the use of a reactive power compensator device, such as the STATCOM.

It is therefore of interest to provide an improved method and system for transforming the AC current to a DC current for use with hydrogen or for transforming a DC current to an AC current for use with a fuel cell.

Summary

It is therefore a goal of the present disclosure to provide a system and a method which may provide a controlled power conversion for use with at least one electrolyzer unit and/or at least one fuel cell.

To achieve this goal, the present disclosure provides a system comprising a converter unit and a control unit, and a method for controlling the system, as defined by the independent claims. Further embodiments are provided in the dependent claims.

According to a first aspect of the present disclosure, a system connectable to an electrical power grid is provided. The system comprises a converter unit comprising a plurality of serially-connected converter cell units. At least one of the plurality of serially-connected converter cell units comprises a first terminal and a second terminal. Said at least one of the plurality of serially-connected converter cell units is dedicated to individually provide a direct current to one electrolyzer unit connected between the first terminal and the second terminal or to individually receive power from at least one fuel cell connected between the first terminal and the second terminal. The system further comprises a control unit configured to control the direct current output from said at least one of the plurality of serially-connected converter cell units or to control the power received from said at least one fuel cell based on at least a reference value.

According to a second aspect of the present disclosure, a method for controlling a system is provided. The system comprises a converter unit comprising a plurality of serially-connected converter cell units. At least one of the plurality of serially-connected converter cell units comprises a first terminal and a second terminal. The at least one of the plurality of serially- connected converter cell units is dedicated to individually provide a direct current to one electrolyzer unit connected between the first terminal and the second terminal or to individually receive power from at least one fuel cell connected between the first terminal and the second terminal.

The method comprises receiving a reference value for driving the electrolyzer unit, said reference value being indicative of a target value for a direct current output between the first terminal and the second terminal or a reference value for receiving power from said at least one fuel cell, said reference value being indicative of a target value for a direct current input between the first terminal and the second terminal. The method further comprises controlling the at least one of the serially-connected converter cell units based on the received reference value.

By reference value it is herein meant a control value to be used for operating the electrolyzer unit and/or the at least one fuel cell. A reference value may for example be the volume or amount of hydrogen to be produced by the electrolyzer unit or consumed by the at least one fuel cell or the rate at which the electrolyzer unit is intended to produce hydrogen or the rate at which the at least one fuel cell is intended to produce power, e.g. by consuming hydrogen (if it is a fuel cell based on hydrogen consumption). Such a reference value may for example be indicative or at least representative of a target value to be used for operation of the at least one of the plurality of serially-connected converter cell units in order to achieve the reference value. The target value derived from the reference value may for example be the level of direct current to be output or input between the first terminal and the second terminal of the at least one of the plurality of serially- connected converter cell units.

The first and second terminals of the at least one of the plurality of serially-connected converter cell units may also be referred to as the positive pole and the negative pole, or the positive terminal and the negative terminal of the at least one of the plurality of serially-connected converter cell units.

In the present embodiments, a converter cell unit of the plurality of serially connected converter cell units is configured, or dedicated, to individually provide power to one (e.g., a single) electrolyzer unit or to receive power from the at least one fuel cell. This means that the converter cell unit can, on its own, drive the operation of the electrolyzer unit or the at least one fuel cell connected to it. In some embodiments, every converter cell unit of the converter unit may be connected to one electrolyzer unit. In some embodiments, every converter cell unit of the converter unit may be connected to at least one fuel cell. In some other embodiments, at least some of the converter cell units of the converter unit may be connected to one electrolyzer unit/fuel cell and the remaining converter cell units may either be unconnected or connected to other types of components (such as batteries or other fuel cells/electrolyzer units as will be further described in more detail below). In some embodiments, some of the converter cell units may be connected to electrolyzer units and some of the converter cell units may be connected to fuel cells. In particular, a fuel cell connected to a converter unit may be operated to consume hydrogen earlier produced by electrolyzer units connected to other converter cell units. Hence, instead of providing a connection of an electrolyzer unit or a fuel cell across a branch of converter cell units, it is proposed in the present disclosure to provide connection of an electrolyzer unit or at least one fuel cell at the level of a converter cell unit of the plurality of serially-connected converter cell units forming the converter unit.

In some embodiments, the electrolyzer unit or the fuel cell may be connected across several of the serially connected converter cell units. There may for example be two, three, four or more of the serially connected converter cell units that together drive the electrolyzer unit. In other words, in one example, two serially connected converter cell units from one branch may be connected to drive a single electrolyzer unit. In another example there may be three converter cell units from three different branches, each branch having serially connected converter cell units, that are connected to drive a single electrolyzer unit. This may be advantageous in that it allows a better match between the converter cell rating and the electrolyser unit rating.

Arranging, or distributing, the converter cell units used to drive a plurality of electrolyzer units in the form of a converter unit in which a converter cell unit is configured to operate one electrolyzer unit or (to cooperate with) one fuel cell is beneficial for reducing the footprint of the system, thereby rendering the system more compact, since no, or at least less, intermediate transformers, passive filters or STATCOMs are needed.

Accordingly, there are provided a system and a method with greater flexibility. Further, the present system and method provide a first function to convert, with the converter unit, an alternating current received from an electrical power grid to a direct current output to at least one electrolyzer unit of the plurality of serially connected converter units or reversed for the at least one fuel cell (e.g. a current input from the fuel cell that is converted from DC to AC for input in the electrical power grid). However, the system may be connected to a DC electrical power grid such that the converter unit is a DC/DC converter unit. In other words, the converter unit may in some embodiments be an AC/DC converter unit and in other embodiments be a DC/DC converter unit.

The amount of hydrogen produced via electrolysis is expected to increase within the next few years. Thus, having more flexible, and even more reliable, systems (and methods) for providing power to electrolyzers for producing hydrogen is of interest. Also more flexible, and even more reliable, systems (and methods) for providing power to the electrical power grid through fuel cells is of interest. In particular, when the converter unit is an AC/DC converter unit, the present system provides advantages over other systems since the use of a plurality of serially-connected converter cell units does not pollute the electrical power grid with harmonics, or at least significantly reduces the occurrence of harmonics on the AC side. Further, compared to prior art systems based on the use of thyristor rectifiers, the need of harmonic filters on the AC side is reduced, and possibly even suppressed. Further, the system and method provide the possibility of controlling reactive power in the electrical power grid. The fast regulation of the reactive power together with the system's possibility to control the active power to the electrolyzer units can be used to stabilize the electrical grid with a high degree of renewables. Similarly, the system may control the active power provided to the electrical power grid by the at least one fuel cell to stabilize the electrical grid, in particular when the renewable energy has low production. Thus the need of added passive or active reactive power compensation may be reduced, and possibly even suppressed.

Furthermore, since different electrolyzers age differently and may have different reference values for the current or voltage needed to produce a specific amount of hydrogen, it may be advantageous to provide a system where one converter cell unit may be used with one electrolyzer unit. This ensures that each converter cell unit can be controlled in order to deliver an optimal direct current to a specific electrolyzer unit. Further, since the electrolyzer units may have different temperatures during use, i.e. , the current needed may vary, it may be advantageous to have the modularity provided by the present system and method.

The fuel cell may have different reference values for the hydrogen needed to produce a specific current or voltage, therefore it may be advantageous to provide a system where one converter cell unit may be used with one fuel cell. This ensures that each fuel cell may be controlled to deliver an optimal direct current to the converter cell unit. It should be noted that the fuel cell may be used with other fuels than hydrogen such as, for example, methanol or ethanol.

Further, since the grid code, i.e., requirements on connected devices of the electrical power grid, can be fulfilled with the present system, the need of STATCOMs usually connected to the point of common connection is reduced or even vanished.

Further, the system provides low harmonic content of voltage and current on the DC side, which is beneficial for operation of the electrolyzer unit. The high flexibility of the system is further beneficial to handle and adapt the DC voltage or the DC current of the electrolyzer unit over time since electrolyzer units may degrade over their lifetime. Further, the present system and method may also be used to support island grids.

According to an embodiment, said at least one of the plurality of serially-connected converter cell units includes at least a first converter cell unit dedicated to individually provide a direct current to one electrolyzer unit and at least a second converter cell unit dedicated to individually receive power from at least one fuel cell.

By having a first converter cell unit connected to one electrolyzer unit and a second converter cell unit connected to the at least one fuel cell, all of the advantages discussed above may be provided by one converter unit. This allows for example the converter unit to both produce power to the electrical power grid through operation of the fuel cell(s) and to consume power from the electrical power grid through operation of the electrolyzer unit(s). As an example, if there is a surplus of power on the electrical power grid, the system may be controlled to consume power by producing hydrogen via the electrolyzer units. However, if there is a deficit of power on the electrical power grid, the system may be controlled to produce power by operating the fuel cells and for example by consuming hydrogen via the fuel cells. The system may therefore be beneficial in connection to renewable energy plants with intermittent production.

The fuel-cells may use hydrogen from a hydrogen storage or a hydrogen pipeline. For example, in general, the fuel-cells may generate electrical energy by using hydrogen earlier produced by the electrolyzer units connected to converter cell units of the converter unit.

According to an embodiment, the control unit is configured to control activation of the electrolyzer unit or the at least one fuel cell via said at least one of the plurality of serially-connected converter cell units based on said at least a reference value. For example, the control unit may be configured to control activation of the electrolyzer unit or the at least one fuel cell via said at least the first converter cell unit or said at least the second converter cell unit. Hence, the control unit may determine, based on the reference value, which of the electrolyzer unit and/or the fuel cell need to be active. The control unit may activate the one or more electrolyzer units or the one or more fuel cells based on the reference value to either trigger power consumption to, or power production from, the electrical power grid, thereby rapidly adapting to the needs of the electrical power grid.

According to an embodiment, at least one of the plurality of serially- connected converter cell units is configured to supply/receive power to/from at least one battery. In the present embodiment, active power from the battery, or fuel-cell, can be used to energize the electrical power grid. This may be advantageous for stabilizing the electrical power grid and possibly also in case of a power outage. The power received from the battery can for example be used to smoothen out the electrolyzer units’ power consumption from the electrical power grid in case an electrolyzer unit with high bandwidth is used, i.e. , the response time when tracking the reference value may be reduced. It may further be used to buffer the power difference due to slow varying electrolyzer units. Further, batteries may be advantageous since they may be used to improve the fast frequency response functionality of the system. The active power from the battery or fuel-cell can be used together with the reactive power of the converter unit to handle a grid with 100% renewables in an island or connected via a weak link to the rest of the power system.

Any batteries may be used, such as for example Li-ion battery racks. Further, any fuel-cell may be used with the system. Further, a system may have more than one converter cell unit connected to a battery, and/or more than one converter cell unit connected to a fuel-cell.

According to an embodiment, the control units is configured to control the power received from said at least one battery or said at least one fuel-cell via the corresponding one of the plurality of serially-connected converter cell units in order to control an active power and/or a reactive power of the electrical power grid. Controlling the active power and/or reactive power of the electrical power grid using the batteries and/or fuel-cells in the system is advantageous in many aspects, as previously stated. In case of a power outage, the system may still provide active power to the electrical power grid. Further, the electrolyzer power consumption can be leveled out using power from the batteries. The control unit may, by controlling active power of the electrolyzer units, fuel cells and/or batteries, improve the fast frequency response functionality of the system.

According to an embodiment, the system further comprises a by-pass switch configured to by-pass the at least one of the plurality of serially- connected converter cell units. The present embodiment is advantageous since it allows for by-passing of malfunctioning converter cell units and/or bypassing of converter cell units connected to malfunctioning electrolyzer units or fuel cells. In case one of the converter cell units is malfunctioning, bypassing the specific (malfunctioning) converter cell unit may be advantageous so that the remaining system may still be used normally. In such case, as the voltage across the converter unit is shared between (or divided among) the serially-connected converter cell units, bypassing one or more of the converter cell units will increase the voltage across each of the converter cell units remaining under operation. The control unit may therefore also be configured to regulate the voltage across a branch of the converter unit in case a bypass switch is activated.

According to an embodiment, the at least one electrolyzer unit includes at least one electrolyzer stack comprising a plurality of electrolyzer cells connected in series. The electrolyzer stacks and cells may for example be based on alkaline, proton exchange membrane (PEM) or solid oxide technologies. Advantages of an alkaline electrolyzer unit are, compared to other types of electrolyzer units, that they comprise cheaper catalysts, and have a higher lifespan. Advantages of a PEM electrolyzer unit, compared to other electrolyzers, are that they have a higher current density, are more compact, have a smaller footprint, have a faster response, and allow for a more dynamic operation.

According to an embodiment, the reference value is a value for driving the electrolyzer unit and/or the reference value is indicative of at least one of an amount of hydrogen to be produced by the at least one electrolyzer unit, a current to be conducted through the at least one electrolyzer unit, a voltage applied to the at least one electrolyzer unit, and a condition of the at least one electrolyzer unit. The reference value may be any relevant information received at the control unit and indicative of a voltage or current needed to drive the electrolyzer unit.

According to an embodiment, the reference value is based on at least one parameter of the electrical power grid. The system may be connected to the electrical power grid such that the reference value may be different grid parameters that are used to balance the electrical power grid. The grid parameter may for example be the frequency or the voltage of the electrical power grid. The reference value may also be based on the active or reactive power balance of the electrical power grid. This allows the system to rapidly react on changes in the electrical power grid, thereby providing stability to the electrical power grid.

According to an embodiment, the at least one of the plurality of serially-connected converter cell units comprises one converter cell with a full bridge topology or a half bridge topology. The converter cell may include any self-commutated semiconductor switches. These self-commutated semiconductor switches include at least insulated-gate bipolar transistors (IGBTs), integrated gate-commutated thyristors (IGCTs), injection-enhanced gate transistors (lEGTs), gate turn-off thyristors (GTOs), and metal-oxide- semiconductor field-effect transistors (MOSFETs).

According to an embodiment, the at least one of the plurality of serially- connected converter cell units comprises a plurality of converter cells connected in parallel, wherein at least one of the plurality of converter cells has a full bridge topology.

According to an embodiment, at least one of the plurality of serially- connected converter cell units comprises a DC/DC converter arranged between the first terminal and the second terminal. The DC/DC converter may be placed between a converter cell unit and an electrolyzer unit. The DC/DC converter may also be placed between a converter cell unit and a battery or fuel-cell. The DC/DC converter may be any conventional DC/DC converter, it may for example be a step-down DC/DC converter. The DC/DC converter may for example be a Buck converter. The DC/DC converter may also be a solid-state transformer (SST) DC/DC converter. The DC/DC converter may, in case it includes a transformer such as for example an SST DC/DC converter, galvanically isolate the converter cell unit from the electrolyzer unit, battery or fuel-cell.

In some embodiments, the electrolyzer unit or the fuel cell may be connected across several of the serially connected converter cell units where each of the converter cell units have a DC/DC converter connected between the converter cell unit and the electrolyzer unit or the fuel cell. The DC/DC converter provide galvanic insulation. Said several of the serially connected converter cell units may be from same branch of serially connected converter cell units or from different branches of serially connected converter cell units. There may for example be two, three, four or more of the serially connected converter cell units each having a DC/DC converter that together drive the electrolyzer unit.

According to an embodiment, the control unit is configured to control a direct current, a direct power or a direct voltage output from the DC/DC converter to one electrolyzer unit based on the reference value or to control a direct current, a direct power or a direct voltage input to the DC/DC converter from one fuel cell based on the reference value. In case a DC/DC converter is used, it may be beneficial to control the output from the DC/DC converter to ensure that the electrolyzer unit receives a steady current. The current can be based on the reference value.

According to an embodiment, the at least one of the plurality of serially connected converter cell units comprises an auxiliary DC/DC converter or an auxiliary DC/AC converter arranged between the first terminal and the second terminal to provide auxiliary power. The auxiliary DC/DC converter or DC/AC converter may provide auxiliary power to an equipment connected to the converter cell. The equipment may for example be a power sensor and/or an electrolyzer, a fuel cell or a battery.

There may for example be a DC/DC converter connected directly between the electrolyzer unit, or the fuel cell, and the converter cell unit and an auxiliary DC/AC converter connected between the first terminal and the second terminal and configured to provide auxiliary power.

According to an embodiment, the system further comprises a transformer connectable to the electrical power grid for galvanically isolating the system from the electrical power grid and for adapting an input voltage level associated with an alternating current received from the electrical power grid. The converter unit may be a chain-link converter unit comprising at least one chain-link branch connected to an AC line of the transformer, wherein the at least one chain-link branch comprises the plurality of serially-connected converter cell units and an inductor. The system described in the present embodiment is an example of a topology in which the converter cell unit may be arranged. The converter cell unit may be used in other topologies as well, for example in a modular multilevel converter, MMC.

A chain-link branch may be referred to as a chain-link arm or similar. Usually, each phase may have one corresponding chain-link branch connected to it.

According to an embodiment, each of the three chain-link branches extends from one individual AC line of the transformer to a common coupling point. Alternatively, according to an embodiment, each of the chain-link branches is connected, at a first end, to a respective AC line and, at a second end to another AC line and to another chain-link branch such that the three chain-link branches are connected in series. The first alternative includes a Y- connection while the second alternative includes a D-connection or a Deltaconnection. Other connections of the converter branches may be possible.

According to an embodiment, said at least one of the plurality of converter cell units is placed on an electrically insulated platform. The electrically insulated platform may provide electrical isolation for the converter cell units, thereby reducing disturbances such as noise and also increasing the lifespan of the converter cell unit. Providing an electrically insulated platform is beneficial since the converter cell may experience varying (sometimes very high) voltage levels and may be prevented from being in connection with ground.

According to an embodiment, said at least one of the plurality of converter cell units, when dedicated to individually provide a direct current to one electrolyzer unit, includes electrically insulated gas and/or electrically insulated water pipes for connection of said one electrolyzer unit.

According to an embodiment of the second aspect, the at least one of the plurality of serially-connected converter cell units comprises a DC/DC converter arranged between the at least one of the plurality of serially- connected converter cell units and the electrolyzer unit or said at least one fuel cell. The step of controlling the at least one of the serially connected converter cell units based on the received reference value includes controlling the DC/DC converter to provide a direct current output to the electrolyzer unit matching the target value or to receive a direct current input from said at least one fuel cell matching the target value, respectively.

According to an embodiment of the second aspect, the system further comprises at least one battery connected to one of the plurality of serially- connected converter cell units. The method further comprises controlling the at least one of the plurality of serially-connected converter cell units that is connected to the at least one battery to control an active power or reactive power of the electrical power grid.

Other objectives, features and advantages of the disclosed embodiments will be apparent from the following detailed disclosure as well as from the drawings.

It is noted that embodiments of the present disclosure relate to all possible combinations of features recited in the claims. Further, it will be appreciated that the various embodiments described for the system as defined in accordance with the first aspect and the embodiments described for the method according to the second aspect are all combinable with each other.

Brief description of the drawings

This and other aspects of the present disclosure will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the disclosure.

Figs. 1 a, 1 b and 2-4 schematically show systems according to exemplifying embodiments of the present disclosure.

Fig. 5a schematically shows a converter cell unit that may be used in a system according to an exemplifying embodiment of the present disclosure.

Figs. 5b, 5c, 5d, 5e, 5f and 5g schematically show converter cell units connected to electrolyzer units or fuel cells that may be used in systems according to exemplifying embodiments of the present disclosure.

Figs. 6-8 schematically show systems according to other exemplifying embodiments of the present disclosure.

Fig. 9 shows a flowchart of a method according to an exemplifying embodiment of the present disclosure.

Detailed description

Figures 1 a and 1 b schematically show systems 100’ and 100 according to two exemplifying embodiments of the present disclosure. Figures

1a and 1 b illustrate two different converter topologies in which the present inventive concept may be implemented. However, it should be noted that the systems 100 and 100’ may be implemented in other converter topologies having a plurality of serially-connected converter cell units.

In Figure 1a, the system 100’ comprises a converter unit 120’ that is connected to an electrical power grid 10T. The electrical power grid 10T may be understood as being part of a local, regional, national, or international electrical power grid, to which the system 100’ is connected. The electrical power grid 10T may for example deliver an excess power created by renewable energy sources such as wind or solar power for saving the excess energy as hydrogen for later use. The electrical power grid 10T in the current embodiment has three connection lines for three phases. It is also conceivable that the system 100’ is used in other applications with an electrical power grid 10T with less, or more phases. The converter unit 120’ has converter arms with a plurality of serially-connected converter cell units 140’. Each of the plurality of serially-connected converter cell units 140’ has a first terminal 147’ and a second terminal 149’ between which an electrolyzer unit 160a’ or a fuel cell 160b’ is connected. However, in other embodiments only one or some of the plurality of serially-connected converter cell units may be connected to an electrolyzer unit or a fuel cell. If an electrolyzer unit 160a’ is connected between the first terminal 147’ and the second terminal 149’, the converter cell unit 140’ is dedicated to individually provide the electrolyzer unit 160a’ with a direct current. If a fuel cell 160b’ is connected between the first terminal 147’ and the second terminal 149’, the converter cell unit 140’ individually receives power from the fuel cell 160b’. The first terminal 147' and the second terminal 149’ may also be referred to as a positive pole 147’ and a negative pole 149’. The system 100’ is controlled by a control unit 170’ that controls how much power the electrolyzer unit 160a’ consumes from the electrical power grid 10T or how much power the fuel cell 160b’ provides to the electrical power grid 10T. The control unit 170’ will be further described in connection to the description of the converter topology of Figure 1 b.

In Figure 1 b, a system 100 having similar features as the system 100' described with reference to Figure 1a is illustrated. However, the system 100 has another topology. Further, Figure 1 b illustrates that the system 100 may comprise a transformer 110 connectable to an electrical power grid 101 for galvanically isolating the system 100 from the electrical power grid 101 and for adapting an input voltage level associated with an alternating current received from the electrical power grid 101 .

In the system 100 shown in Figure 1 b, the converter unit comprises a chain-link converter unit 120. The chain-link converter unit 120 comprises three chain-link branches 130 each coupled to one individual AC line of the transformer. The system 100 could however have more or fewer chain-link branches 130. In other words, in general, the chain-link converter unit 120 may comprise at least one chain-link branch 130. Each of the chain-link branches 130 comprises a plurality of serially-connected converter cell units 140 and an inductor 150. In Figure 1 , each of the serially-connected converter cell units 140 comprises a first terminal 147 and a second terminal 149. The converter cell unit 140 may provide a direct current output between the first terminal 147 and the second terminal 149. In Figures 1a and 1 b, the plurality of serially-connected converter cell units 140 are connected to either one electrolyzer unit 160a or a fuel cell 160b. Further, in Figures 1a and 1 b, each of the plurality of serially-connected converter cell units 140 is dedicated to individually provide a direct current to one electrolyzer unit 160a connected between the first terminal 147 and the second terminal 149 of that converter cell unit 140 or to individually receive power from at least one fuel cell 160b connected between the first terminal 147 and the second terminal 149. In Figure 1 b, each chain-link branch 130 comprises six serially-connected converter cell units 140, each connected to one electrolyzer unit 160a or at least one fuel cell 160b. The system shown in Figure 1 b is provided as an example and the number of converter cell units 140 in each branch may be different between the branches 130 and it may also be envisaged that not each converter cell unit 140 is coupled to an electrolyzer unit 160a or to a fuel cell 160b. For example, some of the converter cell units 140 may be uncoupled or connected to other components, such as batteries, or to combinations of electrolyzer units and fuel cells as will be described in relation to Figures 6-8. It should be stressed that the converter unit 120 may be of any topology with a plurality of serially-connected converter cell units 140, not only the topologies shown in the embodiments described in connection to Figures 1a and 1b. As already mentioned, there may be converter units 120 that are adapted for fewer or more phases, which may also benefit from the present inventive concept.

The system, in Figure 1 b, further comprises a control unit 170 configured to control the direct current output from at least one of the plurality of serially-connected converter cell units 140 (i.e. the current output between the first terminal 147 and the second terminal 149 of that converter cell unit) based on a reference value for driving the electrolyzer unit 160a coupled to that converter cell unit 140. When a fuel cell is connected between the first terminal 147 and the second terminal 149, the control unit 170 is configured to control the power received from said at least one fuel cell 160b based on at least a reference value to that converter cell unit 140. The control unit 170 may further be configured to control a plurality of converter cell units 140 based on a plurality of reference values obtained for the electrolyzer units 160a connected to the plurality of converter cell units and/or configured to control a plurality of converter cell units 140 based on a plurality of reference values obtained for the fuel cells 160b.

Alternatively, the control unit may receive a single reference value for driving the plurality of electrolyzer units and determine a plurality of target values for driving the plurality of electrolyzer units based on the received single reference value. The control unit may as a result determine control signals for operating the converter cell units associated to the plurality of electrolyzer units.

Similarly, the control unit may receive a single reference value representative of a requirement of the electrical power grid. Based on this reference value, the control unit may determine a plurality of target values for driving the plurality of electrolyzer units connected to converter cell units of the system and/or for operating fuel cells connected to converter cell units of the system. In other words, the control unit may determine whether only electrolyzer units, only fuel cells or a combination of electrolyzer units and fuel cells need to be activated in order to achieve the reference value. The control unit may as a result determine and then transmit control signals for operating the converter cell units of the converter unit.

A reference value may be indicative of an amount of hydrogen to be produced by the electrolyzer unit 160a associated with this reference value or as mentioned above, in case of a single reference value, may be indicative of an amount of hydrogen to be produced by a plurality of electrolyzer units. The reference value may also control a level of power required by the electrical power grid from the fuel cells connected to the converter cell units, such as for example control an amount of hydrogen that the fuel cells should consume.

The reference value may also, or alternatively, be indicative of a current to be conducted through the electrolyzer unit(s) 160a. Further, the reference value may be indicative of a voltage applied to the electrolyzer unit(s) 160. Also, the reference value may be indicative of a condition of the electrolyzer unit(s) 160. In other words, in general, the reference value may be any relevant information received at the control unit indicative of a voltage or current needed to drive the electrolyzer unit 160.

Furthermore, the reference value may be based on at least one parameter of the electrical power grid. The grid parameter may for example be the frequency or the voltage of the electrical power grid. The reference value may also be based on the active or reactive power balance of the electrical power grid. This allows the system to rapidly adapt to changes in the electrical power grid.

In Figure 1 b, the inductor 150 of each chain-link branch 130 is placed between the AC line and the converter cell units 140 of each chain-link branch. It is however plausible to have other placements for the inductors 150. For example, instead of using a single inductor, the inductance may be distributed to the plurality of serially-connected converter cell units 140 connected to the system, such as for example described in connection to Figure 3. The inductor 150 may also be referred to as a branch reactor 150 and may be any type of inductor or branch reactor.

The three chain-link branches 130 in Figure 1b extends from one individual AC line of the transformer to a common coupling point 102. This configuration corresponds to a Y-connection. Other connections of the chainlink branches may be possible, such as for example via a Delta-connection as shown in Figure 2.

The plurality of serially-connected converter cell units 140 may comprise different components. As an example, each converter cell unit 140 may comprise a plurality of converter cells connected in parallel. At least one of the plurality of converter cells connected in parallel may have a full bridge topology. In some embodiments, all of the plurality of converter cells connected in parallel may have a full bridge topology. In other embodiments, each converter cell unit may have a half bridge topology. In yet a further embodiment, the converter unit may include a mix of cells having a halfbridge topology and full-bridge topology.

As another example, each converter cell unit 140 may comprise only one converter cell with a full bridge topology. Each chain-link branch 130 may also comprise a mixture of converter cell units 140 with one or more converter cells. In other embodiments, the converter cell units may be of half bridge topology.

Further, at least one of the converter cell units 140 may comprise a DC/DC converter (as further described in more detail with reference to Fig. 5d). The DC/DC converter may be connected between the converter cell unit 140 and the electrolyzer unit 160a or, alternatively, between the converter cell unit 140 and the fuel cell 160b. The DC/DC converter may be any general DC/DC converter, it may for example be a step-down DC/DC converter. In some embodiments, there may be an auxiliary AC/DC converter, which is configured to provide auxiliary power, connected to the converter cell unit 140 between the first terminal 147 and the second terminal 149.

The system 100 may further comprise one or a plurality of by-pass switch(es) configured to by-pass one or some of the plurality of serially- connected converter cell units 140 (as further described in more detail with reference to Fig. 5c). A by-pass switch may be activated to by-pass a malfunctioning converter cell unit 140 or a converter unit upon which a malfunctioning electrolyzer unit 160a or a malfunctioning fuel cell 160b is connected.

The transformer 110 may be any standard transformer within the present technological field. The transformer 110 may for example be a two- winding transformer 110 per phase. It is however possible to have a three- winding transformer 110 connected to two chain-link converter units 120.

The physical size of the system 100 may be restricted by the size of the electrolyzer units 160a or fuel cell 160b and depending on the physical size of the components used in the system 100, the physical size of the system 100 may vary.

Figure 2 schematically shows a system 200 according to another exemplifying embodiment of the present disclosure.

The system 200 of Figure 2 is equivalent or at least similar to the system 100 described with reference to Figure 1 b except that the connection between the three chain-link branches 230 is different. In the system 200, each chain-link branch 230 is connected, at a first end, to a respective AC line and, at a second end opposite to the first end, to another AC line and to another chain-link branch 230 such that the three-chain link branches 230 are connected in series. The connection corresponds to a D-connection or Deltaconnection.

To reduce noise and disturbances one of the plurality of converter cell units 240 that is connected to an electrolyzer unit 260a or a fuel cell 260b is placed on an electrically insulated platform 242. All of the converter cell units

240 may be positioned on electrically insulated platforms. The converter cell units of the other exemplifying embodiments may also be positioned on electrically insulated platforms, the electrically insulated platform 242 in Figure 2 is an example of how it may be used.

Figure 3 schematically shows a system 300 according to an exemplifying embodiment of the present disclosure.

The system 300 of Figure 3 is equivalent or at least similar to the system 100 described with reference to Figure 1 b except that the system 300 comprises a plurality of inductors 350. In the system 300, some of the inductors 350 are arranged between two consecutively arranged converter cell units 340 of the plurality of serially-connected converter cell units 340. Such an arrangement of the inductors 350 provides an improved handling of internal ground faults. It may also be envisaged a system 300 with inductors 350 placed between only some of the converter cell units 340.

Figure 4 schematically shows a system 400 according to an exemplifying embodiment of the present disclosure.

The system 400 of Figure 4 is equivalent or at least similar to the system 100 described with reference to Figure 1 b except that the system 400 further comprises a plurality of filters 425. The filters 425 may be RC-filters comprising at least one resistor and at least one capacitor. A filter 425 may also include at least one power electronic device configured to filter harmonics. A filter 425 may also be a combination of an RC-filter and power electronic devices. In general, a filter 425 may be one of, or a combination of, a passive filter and an active filter. A filter 425 is electrically connected in parallel with a respective converter cell unit 440 between the first terminal 447 of the converter cell unit and its second terminal 449, as shown in Fig. 4. In some example systems, only some of the serially-connected converter cell units 440 may have filters 425 coupled to them. Further, a filter 425 may be replaced with a surge arrestor in order to limit overvoltage. It is also envisioned to have both a filter 425 and a surge arrestor coupled between at least one converter cell unit and an electrolyzer unit 460a or a fuel cell 460b.

Figure 5a schematically shows a converter cell unit 540a that may be used in a system according to any exemplifying embodiment of the present disclosure, such as for example any system described with reference to Figures 1a, 1b or 2-4.

In certain embodiments of the present disclosure, the converter cell units 540a may comprise a single converter cell 544 having a full bridge topology as shown in Figure 5a. The converter cell unit 540a in Figure 5a comprises a converter cell with a first to fourth semiconductor switch 545a, 545b, 545c, 545d that are connected in a full-bridge configuration. The converter cell 544 further comprises an energy storage which is typically implemented as a capacitor arrangement comprising at least one capacitor 542. The capacitor 542 is configured to store electrical energy and thereby provide a voltage. The semiconductor switches 545a, 545b, 545c, 545d may include any self-commutated semiconductor switches. These selfcommutated semiconductor switches include at least insulated-gate bipolar transistors (IGBTs), integrated gate-commutated thyristors (IGCTs), injection- enhanced gate transistors (lEGTs), gate turn-off thyristors (GTOs), and metal- oxide-semiconductor field-effect transistors (MOSFETs).

In some embodiments, the converter cell unit 540a may comprise a plurality of converter cells connected in parallel. In that case, at least one of the plurality of converter cells may have a full bridge topology such as shown in Figure 5a.

Figure 5b schematically shows a converter cell unit 540b connected to an electrolyzer unit 560athat may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540b is similar to the converter cell unit 540a described in relation to Figure 5a. The converter cell unit 540b comprises one converter cell 544 with a full bridge topology. An electrolyzer unit 560a or a fuel cell 560b is connected between a first terminal 547 and a second terminal 549 of the converter cell unit 540b. The electrolyzer unit 560a may be any electrolyzer unit and may for example be at least one electrolyzer stack comprising a plurality of serially-connected electrolyzer cells.

Figure 5c schematically shows a converter cell unit 540c connected to an electrolyzer unit 560a or a fuel cell 560b that may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540c is similar to the converter cell unit 540b described in relation to Figure 5b. The converter cell unit 540c further comprises a by-pass switch 541 configured to by-pass the converter cell unit 540c. In case the converter cell unit 540c or the electrolyzer unit 560a or the fuel cell 560b malfunctions, the by-pass switch may be activated (e.g., by the control unit) to bypass the converter cell unit 540c. Since the system comprises a plurality of serially-connected converter cell units, malfunctioning converter cell units may advantageously be bypassed to make sure that the remaining converter cell units may still be used.

In Figure 5c where the converter cell unit is dedicated to individually provide a direct current to one electrolyzer unit 160a, the converter cell unit may include electrically insulated gas 582 and/or electrically insulated water pipes 580 connected to the electrolyzer unit 160a. The electrically insulated gas pipes 582 and the electrically insulated water pipes 580 may be used in other converter cell units and may form part of a system according to any exemplifying embodiment of the present disclosure. In other embodiments, the fuel cell may also have insulated pipes.

Figure 5d schematically shows a converter cell unit 540d connected to an electrolyzer unit 560a or the fuel cell 560b that may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540d is similar to the converter cell unit 540b described in relation to Figure 5b. The converter cell unit 540d further comprises a DC/DC converter 543. The DC/DC converter 543 may for example be a step-down DC/DC converter 543. The DC/DC converter 543 is connected between the converter cell 544 of the converter cell unit 540d and the electrolyzer unit 560a or the fuel cell 560b. A control unit, as described throughout the present description, may in this example control a direct current output from the DC/DC converter 543 to the electrolyzer unit 560a based on a reference value for driving the electrolyzer unit or control a direct current input to the DC/DC converter 543 from the at least one fuel cell 560b based on a reference value. The converter cell unit may also comprise an auxiliary converter 546, such as for example a DC/DC converter, that is configured to provide auxiliary power between the first terminal 547 and the second terminal 549 to an equipment such as for example a power sensor and/or an electrolyzer unit, a fuel cell, or a battery. The auxiliary converter 546 may in some other embodiments be a DC/AC converter. The converter cell unit 540 may comprise both the DC/DC converter 543 and the auxiliary DC/DC converter or DC/AC converter 546, as shown in Figure 5d, or in other embodiments it may only comprise one of the two converters 543, 546.

Figure 5e schematically shows a converter cell unit 540e connected to an electrolyzer unit 560a or to a fuel cell 560b that may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540e comprises two converter cells 544, with full bridge topology, coupled in parallel. This may be advantageous in case a higher voltage or current is needed to drive the electrolyzer unit 560a. In the embodiment disclosed in Figure 5e, both converter cells 544 have a full bridge topology. In this embodiment, as can be seen in Figure 5e, each converter cell 544 is coupled to one inductor 550. In case converter cell units 540 as the one described in Figure 5e would be used in a system as disclosed in this application, for example the system 300 disclosed in Figure 3, each inductor 350 may be interchanged for two inductors 550 or the two inductors 550 may be used in addition to the inductors 350 for each converter cell unit 540.

Figure 5f schematically shows a converter cell unit 540f connected to an electrolyzer unit 560a or to a fuel cell 560b that may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540f is similar to the embodiment of Figure 5e since it may provide a higher power output to the electrolyzer unit 560a than a converter cell unit as disclosed in, for example, Figure 5b. The converter cell unit 540f of Figure 5f has more semiconductor switches compared to the embodiment disclosed in Figure 5b. In this example, the converter cell unit 540f has eight semiconductor switches. Compared to the configuration shown in Figure 5b, the converter cell unit 540f has two phase legs connected in parallel, both in and out of the converter cell unit 540f, with two semiconductor switches coupled to each leg. Similar to the embodiment described with reference to Figure 5e, each phase leg includes an inductor 550.

Figure 5g schematically shows a converter cell unit 540g connected to an electrolyzer unit 560a or to a fuel cell 560b that may be part of a system according to any exemplifying embodiment of the present disclosure.

The converter cell unit 540g comprises a converter cell with a first semiconductor switch 545a and a second semiconductor switch 545b that are connected in a half bridge configuration. The converter cell 544 further comprises an energy storage which is typically implemented as a capacitor arrangement comprising at least one capacitor 545. The capacitor 545 is configured to store electrical energy and thereby provide a voltage. The semiconductor switches 545a, 545b may include any self-commutated semiconductor switches. These self-commutated semiconductor switches include at least insulated-gate bipolar transistors (IGBTs), integrated gate- commutated thyristors (IGCTs), injection-enhanced gate transistors (lEGTs), gate turn-off thyristors (GTOs), and metal-oxide-semiconductor field-effect transistors (MOSFETs).

The converter cell unit 540g further comprises an optional by-pass switch 541 configured to by-pass the converter cell unit 540g. In case the converter cell unit 540c or the electrolyzer unit 560a or the fuel cell 560b malfunctions as described in connection to the converter cell 540c. Further, the half bridge converter cell unit 540g may also comprise a DC/DC converter and/or an auxiliary DC/AC converter as described in connection to the converter cell unit 540d. A system according to the present disclosure may include converter cell units using one or more of the converter cell units described with reference to Figures 5a-5g.

Figure 6 schematically shows a system 600 according to another exemplifying embodiment of the present disclosure.

The system 600 is similar to the system 100 described with reference to Figure 1 b. The system 600 comprises three chain-link branches 630. Each of the chain-link branches 630 comprises nine serially-connected converter cell units 640. Six of the converter cell units 640 of each chain-link branch 630 are connected to one individual electrolyzer unit 660a while three (other) of the converter cell units 640 of each chain-link branch 630 are connected to batteries 680. The converter cell units 640 that are connected to the batteries may be configured to supply power to the batteries 680 in order to charge the batteries 680. The converter cell units 640 that are connected to the batteries may further be configured to receive power from the batteries 680. The control unit 670 may control the received power from the batteries 680 via the converter cell units 640 in order to control an active power and/or a reactive power of the electrical power grid 601 .

The batteries 680 may be any kind of suitable batteries such as, for example, Li-ion battery racks. In some configurations, as compared to the system 600 shown in Fig. 6, more or fewer converter cell units 640 of the system 600 may be connected to batteries 680. The chain-link branches 630 may comprise an arbitrary number of serially-connected converter cell units 640 and some may be connected to electrolyzer units 660a while others are connected to batteries 680.

Figure 7 schematically shows a system 700 according to an exemplifying embodiment of the present disclosure.

The system 700 is similar to the system 600 described in relation to Figure 6. The system 700 differs in that three converter cell units 740 of each chain-link branch 730 are connected to fuel-cells 760b instead of batteries. The fuel-cells 760b may be any type of fuel-cells 760b and may for example be hydrogen fuel-cells 760b. With fuel-cells 760b electric power can be created from consumption of hydrogen and oxygen. The converter cell units 640 connected to fuel-cells 760b may be configured to receive power from the fuel-cells 760b that can be used to energize the electrical power grid 701 .

The control unit 770 may be configured to control the power received from the fuel-cells 760b via the converter cell units 740 in order to control an active power and/or reactive power of the electrical power grid 701 . The system 700 may comprise an arbitrary number of fuel-cells 760b each connected to a converter cell unit 740. The example in Figure 7 is not meant to be limiting and merely acts as an example on how the chain-link converter unit 720 may be configured.

Figure 8 schematically shows a system 800 according to an exemplifying embodiment of the present disclosure.

The system 800 is similar to the systems described in relation to the previous figures. The system 800 comprises three chain-link branches 830. Each chain-link branch 830 comprises a plurality of serially-connected converter cell units 840. Six of the converter cell units 840 of each chain-link branch 830 are connected to electrolyzer units 860a. Three of the converter cell units 840 of each chain-link branch 830 are connected to fuel-cells 860b and three of the converter cell units 840 of each chain-link branch 830 are connected to batteries 880. A converter cell unit 840 connected to one electrolyzer unit 860 may be dedicated to individually provide a direct current to said one electrolyzer unit 860 connected between a first 847 and a second terminal 849 of the converter cell unit 840. The converter cell units 840 connected to the batteries 880 may be configured to supply/receive power to/from the batteries 880. The converter cell units 840 connected to the fuelcells 890 may be configured to receive power from the fuel-cells 890. The system 800 shown in Fig. 8 is an example of a system and other examples with variations as those described in the other embodiments may be possible. For example, the chain-link branches 830 may be connected in a Deltaconfiguration as in Figure 2 instead of a Y-connection. Further, as an example, the number of chain-link branches 830 may vary, and the number of converter cell units 840 of each branch may also vary.

In Figures 6-8, different combinations of components connected to the plurality of serially-connected converter cell units have been described in a converter unit topology as described in Figure 1 b. However, it should be appreciated that these combinations of electrolyzer units, fuel cells and batteries may be used in other topologies where the converter unit has a plurality of serially-connected converter cell units as well, such as for example the topologies shown in Figure 1a and 2-4.

Figure 9 shows a flowchart of a method 900 according to an exemplifying embodiment of the present disclosure.

The method 900 may be implemented in a control unit configured for controlling a system according to any exemplifying embodiment of the present disclosure, such as those described with reference to the previous Figures.

The method 900 comprises receiving 910 a reference value for driving the electrolyzer unit or for receiving power from the fuel cell. As mentioned above, the reference value may be indicative of a target value for the direct current output between the first terminal and the second terminal. The reference value could alternatively be indicative of a voltage drop across the first and second terminals of the converter cell unit or both the direct current output and the voltage drop. The reference value may be a voltage value or a current value, or an active power value, or a value of hydrogen production/consumption. The reference value may alternatively be an amount of hydrogen to be created by the individual electrolyzer unit. The reference value may alternatively be a collective reference value, i.e. , a value shared by all electrolyzer units connected to system, in which case the control unit may determine an individual reference value for an individual electrolyzer unit based on the collective reference value. The reference value may be any reference value related to the electrolyzer unit that may be indicative of a voltage or current for operating the electrolyzer unit. The reference value may further be any reference value based on at least one parameter of the electrical power grid for controlling the electrolyzer unit and/or the fuel cell based on that parameter. The reference value may therefore also be a collective reference value, i.e. , a value shared by all electrolyzer units and fuel cells connected to system, in which case the control unit may determine an individual reference value for each individual electrolyzer unit and each fuel cell based on the collective reference value.

The method 900 further comprises controlling 920 the at least one of the serially-connecter converter cell units based on the received reference value. In case the received reference value is indicative of a target value for the direct current output, the controlling 920 of the converter cell unit may control the direct current output from the converter cell unit. In case the received reference value is a current value, then the controlling 920 may be performed in order for the system to match the direct current output with the current value.

The method 900 may, in some embodiments, include more steps. For example, in case the at least one of the serially-connected converter cell units comprises a DC/DC converter arranged between the at least one of the plurality of serially-connected converter cell units and the electrolyzer unit, then the step of controlling 920 the at least one of the serially-connected converter cell units based on the received reference value includes controlling 930 the DC/DC converter to provide a direct current output to the electrolyzer unit matching the target value (or to provide auxiliary power to an equipment connected to the converter cell).

In case the system comprises at least one battery or at least one fuelcell connected to one of the plurality of serially-connected converter cell units, the method 900 may further comprise controlling 940 the at least one of the plurality of serially-connected converter cell units that is connected to the at least one battery or the at least one fuel-cell to control an active power or reactive power of the electrical power grid.

While the present invention has been illustrated in the appended drawings and the foregoing description, such illustration is to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the appended claims, the word “comprising” does not exclude other elements or steps, and the indefinite article ”a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1 . A system (100) connectable to an electrical power grid, said system comprising: a converter unit (120) comprising a plurality of serially-connected converter cell units (140), wherein at least one of the plurality of serially- connected converter cell units comprises a first terminal (147) and a second terminal (149), and wherein said at least one of the plurality of serially- connected converter cell units is dedicated to individually provide a direct current to one electrolyzer unit (160a) connected between the first terminal and the second terminal or to individually receive power from at least one fuel cell (160b) connected between the first terminal and the second terminal; and a control unit (170) configured to control the direct current output from said at least one of the plurality of serially-connected converter cell units or to control the power received from said at least one fuel cell based on at least a reference value.

2. The system according to claim 1 , wherein said at least one of the plurality of serially-connected converter cell units include at least a first converter cell unit dedicated to individually provide a direct current to one electrolyzer unit and at least a second converter cell unit dedicated to individually receive power from at least one fuel cell.

3. The system according to claim 1 or 2, wherein the control unit is configured to control activation of the electrolyzer unit or the at least one fuel cell via said at least one of the plurality of serially-connected converter cell units based on said at least a reference value.

4. The system according to any one of the preceding claims, wherein at least one of the plurality of serially-connected converter cell units are configured to supply/receive power to/from at least one battery (680).

5. The system according to claim 4, wherein the control unit is configured to control the power received from said at least one battery or the at least one fuel-cell via the corresponding one of the plurality of serially- connected converter cell units in order to control an active power and/or a reactive power of the electrical power grid.

6. The system according to any one of the preceding claims, further comprising a by-pass switch (541 ) configured to by-pass at least one of the plurality of serially-connected converter cell units.

7. The system according to any one of the preceding claims, wherein the at least one electrolyzer unit includes at least one electrolyzer stack comprising a plurality of electrolyzer cells connected in series.

8. The system according to any one of the preceding claims, wherein the reference value is a value for driving the electrolyzer unit and/or wherein the reference value is indicative of at least one of an amount of hydrogen to be produced by the at least one electrolyzer unit, a current to be conducted through the at least one electrolyzer unit, a voltage applied to the at least one electrolyzer unit, and a condition of the at least one electrolyzer unit.

9. The system according to any one of the preceding claims, wherein the reference value is based on at least one parameter of the electrical power grid.

10. The system according to any one of the preceding claims, wherein at least one of the plurality of serially-connected converter cell units comprises one converter cell (544) with a full bridge topology or a half bridge topology.

11 . The system according to any one of the preceding claims, wherein at least one of the plurality of serially-connected converter cell units comprises a DC/DC converter (543) arranged between the first terminal and the second terminal.

12. The system according to claim 11 , wherein the control unit is configured to control a direct current, a direct power or a direct voltage output/input from/to the DC/DC converter to one electrolyzer unit, or from one fuel cell, based on the reference value.

13. The system according to any one of the preceding claims, wherein at least one of the plurality of serially-connected converter cell units comprises an auxiliary DC/DC converter or an auxiliary DC/AC converter arranged between the first terminal and the second terminal to provide auxiliary power.

14. The system according to any one of the preceding claims, the system further comprising; a transformer (110) connectable to the electrical power grid (101 ) for galvanically isolating the system from the electrical power grid and for adapting an input voltage level associated with an alternating current received from the electrical power grid, and wherein the converter unit is a chain-link converter unit comprising at least one chain-link branch (130) connected to an AC line of the transformer, wherein the at least one chain-link branch comprises the plurality of serially- connected converter cell units (140) and an inductor (150).

15. The system according to any one of the preceding claims, wherein said at least one of the plurality of converter cell units is placed on an electrically insulated platform (242).

16. The system according to any one of the preceding claims, wherein said at least one of the plurality of converter cell units, when dedicated to individually provide a direct current to one electrolyzer unit (160a), includes electrically insulated gas (582) and/or electrically insulated water pipes (580) for connection of said one electrolyzer unit.

17. A method (900) for controlling a system (100) comprising a converter unit comprising a plurality of serially-connected converter cell units, wherein at least one of the plurality of serially-connected converter cell units comprises a first terminal and a second terminal, and wherein the at least one of the plurality of serially-connected converter cell units is dedicated to individually provide a direct current to one electrolyzer unit connected between the first terminal and the second terminal or to individually receive power from at least one fuel cell connected between the first terminal and the second terminal, the method comprising: receiving (910) at least a reference value for driving the electrolyzer unit and indicative of a target value for a direct current output between the first terminal and the second terminal or for receiving power from said at least one fuel cell and indicative of a target value for a direct current input between the first terminal and the second terminal; and controlling (920) said at least one of the serially-connected converter cell units based on the received reference value or based on the at least one parameter of the electrical power grid.

18. The method according to claim 17, wherein the at least one of the plurality of serially-connected converter cell units comprises a DC/DC converter arranged between the at least one of the plurality of serially- connected converter cell units and the electrolyzer unit or said at least one fuel cell, and wherein controlling the at least one of the serially connected converter cell units based on the received reference value, or based on said at least one parameter of the electrical power grid, includes controlling (930) the DC/DC converter to provide a direct current output to the electrolyzer unit matching the target value or to receive a direct current input from said at least one fuel cell matching the target value, respectively.

19. The method according to any one of claims 17-18, wherein the system further comprises at least one battery connected to one of the plurality of serially-connected converter cell units, and wherein the method further comprises: controlling (940) the at least one of the plurality of serially- connected converter cell units that is connected to the at least one battery or the at least one fuel cell to control an active power or a reactive power of the electrical power grid.